An in vitro tissue culture system will be used to continue studies of the effects of potential therapeutic agents and compounds which should help elucidate basic effects of hormones on bone. Substances which competed for glucocorticoid binding sites in bone cell cytosol and for the effects of glucocorticoids on bone protein synthesis will be tested for their antagonism of glucocorticoid inhibition of bone resorption. Vitamin D metabolites and analogs will be studied further. Included in the proposed studies are the development of a bioassay for for 1,25-(OH)2D3 based upon the effects of this compound on bone resorption. Cytosol receptors for 25-OH-D3 and 1,25-(OH)2D3 will be sought and investigated and the antagonism of binding by analogs compared with the effects of the analogs in vitro. The effects of the active principle of Solanum malacoxylon will be compared with actions of the vitamin D congeners. The calcium ionophore A 23187 and other ionophores will be studied further in culture to determine whether an enhancement of calcium permeability alters the response to stimulators on bone resorption, such as PTH and 1,25-(OH)2D3, and inhibitors such as glucagon and calcitonin. The interaction between calcium transport and adenyl cyclase in bone resorption will be investigated using the ionophores. BIBLIOGRAPHIC REFERENCES: Feldman, D., Dziak, R., Koehler, R. and Stern, P.H. Cytoplasmic glucocorticoid binding proteins in bone cells. Endocrinology 96, 29-36, 1975. Dziak, R. and Stern, P.H. Parathyromimetic effect of the ionophore, A23187, on bone cells and organ cultures. Biochem. Biophys. Res. Comm. 65, 1343-1349, 1975.